An undercoating, when referring to painting, is a primer. Primer is applied as an undercoat before the topcoat of paint. The undercoat is used to provide a moisture barrier and a bonding surface for the paint. One of the quickest methods to apply undercoating is to spray it on by using a paint gun attached to an airless sprayer or an HVLP sprayer. The type of spray gun and sprayer you use is based on what you are painting. Airless sprayers and guns emit a high volume of product under high pressure, and HVLP sprayers emit a high volume of product under low pressure.
Cover everything with plastic drops that is in the path of the sprayer. Airless paint sprayers create a lot of over spray, and even though HVLP sprayers are more controlled, there is still over spray.
Prime the pump on the airless sprayer by placing the siphon hose into the bucket of primer and the intake hose into a gallon bucket of water. Turn the knob on the airless sprayer to prime. When the primer begins to draw into the pump, turn the knob on the airless sprayer to the spray setting.
Attach the airless hose to the inlet on the end of the gun handle. Fill the spray cup with primer for an HVLP sprayer. Screw the spray gun to the cup. Attach the HVLP hose to the handle of the spray gun. Best use for the HVLP sprayer is on doors, furniture and anywhere that you need more control of over spray.
Aim the spray gun of either type of sprayer directly at the surface you want to spray. Keep the airless gun at least 18 inches away from the surface you are spraying and the HVLP gun at least 8 inches from the surface. Pull the trigger on the gun, and move your arm to the right at the same time or to the left if you hold the gun with your left hand. Release the trigger when you cannot spray any further. Pull the trigger on the gun in, and bring your arm back to you, spraying down further on the surface you are painting. Overlap each pass that you make with the spray gun by at least 3 inches.
Continue this pattern until you are finished.
Using a spray gun is a matter of continuous motion. Do not stop moving your paint gun until you release the trigger. High volumes of paint are spraying all at once, and it will run. Automotive undercoatings are sprayed with a gun that is specially made for that type of material. This gun is also attached to an HVLP type sprayer.
Keep your hands away from the paint coming out of airless sprayers. It will embed in your skin and cause serious infections.